This invention relates generally to injection molding machines and, more particularly, to injection molding machines including means for selectively opening and closing the hot sprue of a machine molding assembly designed for molding centrally apertured record discs, such as video information discs.
An example of an injection molding machine of this particular type is disclosed in copending and commonly assigned patent applications U.S. Ser. No. 847,367, filed in the name of J. R. Holmes et al., and entitled "METHOD AND MEANS FOR REPLICATING CENTRALLY APERTURED VIDEO DISC RECORDS", and U.S. Ser. No. 031,205, filed in the name of J. R. Holmes, and entitled "MOLDING APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORD DISCS". The apparatus described in these applications comprises an injection molding machine having a molding assembly with first and second mold halves reciprocally movable between a closed position, wherein molten plastic material is injected into an annular mold cavity to form a video information disc, and an open position wherein the mold parting line is opened and the resultant molded video information disc is removed from the machine. The annular mold cavity is defined by a pair of planar, disc-shaped stamping dies secured to platens which are in turn reciprocally movable toward and away from each other along with the respective mold halves.
In injection molding machines of this general type, it is well known to inject molten plastic material under pressure through an injection sprue bushing and into the mold cavity defined by the two disc-shaped stamping dies. When the mold cavity is filled with the molten plastic material, the surrounding machine structure including the respective mold halves and the sprue bushing are cooled by a circulating liquid coolant to reduce the temperature of the molten plastic material and thereby solidify the material. In this manner, the molded record disc is quickly solidified within the mold cavity whereupon the solidified disc can be removed from the machine and the cycle restarted, thereby allowing a rapid production rate of the record discs. However, the plastic material within the sprue bushing is relatively thick compared with the plastic material within the mold cavity, whereby the plastic material within the sprue bushing takes the longest time to solidify. The cycle time for molding the record discs is therefore dependent upon the cooling time of the sprue material, and not upon the cooling time of the disc per se. Moreover, this solidified sprue material must be removed from the molded record disc, as by means of a punch assembly or the like, to yield the desired thin, substantially planar record disc including the desired central aperture.
The present invention provides an injection molding machine including an improved mold assembly particularly designed for use in making relatively thin, substantially planar record discs, such as video information discs, wherein the disc production rate is substantially independent of the solidification time of molten plastic material in the sprue bushing region of the injection machine. The present invention fulfills this need by providing a valve assembly for isolating the molten plastic material within the sprue bushing from the mold cavity during solidification of the plastic material.